In-situ combustion process



Feb. 23, 1965 B. T. wlLLMAN 3,170,515

IN-sITu coMBUsTIoN PRocEss Filed June 12. 1961 cuMuLATwE Gon MscF/BBL I I 0 20 40 60 8O IOO RECOVERY L OIL IN PLACE FIG. l

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BY Mi" 4' ATTORNEY United States Patent O "ice mm., ,..IZQIG 3,170,515 lN-SITU COMBUSTION PROCESS Bertram T. Willman, Tulsa, Okla., assignor to Jersey Production Research Company, a corporation of Delaware Filed June 12, 1961, Ser. No. 116,452 4 Claims. (Cl.` 166-11) `The present invention is concerned with a thermal method for the recovery of petroleum from subterranean reservoirs. The invention is more particularly related to a unique technique for recovering oil from subterranean oil reservoirs by in-situ combustion used in conjunction with a dense water phase. The invention is especially concerned with a two phase method wherein a bank of water with a density greater than the oil contained in a reservoir is injected into the reservoir in a first phase prior to initiating combustion in a second phase. Both injection and production wells are preferentially completed in the bottom part of an oil containing formation. The water, since it is more dense than the crude oil, remains in and channels along the bottom of the reservoir. Thus, when burning is initiated, the water saturation near the base of the reservoir provides a path of high mobility for the air and combustion products. As a consequence of this, burning develops 4down the center of a vertical section of the reservoir instead of at the top as would occur if a bank of dense water is not utilized.

In the .recovery of oil from subterranean reservoirs, there have been substantial advances in recovery methods so as -to substantially increase the recovery of oil. However, appreciable quantities of oil remain in reservoirs after the termination of existing methods and in spite of such advances. Indeed, it is estimated that only about 3() 4to 50 percent of the oil in most reservoirs can be economically recovered by present techniques. Thus, there continues to exist a great interest in improving oil recovery methods.

One class of methods for recovering oil from underground reservoirs comprises in-situ combustion methods. These methods in one ,embodiment involve the establishment of a combustion front within a reservoir in the vicinity of one or more injection wells and the subsequent introduction of a combustion-supporting gas behind the combustion front in order to move the combustion front through the reservoir toward one or more production wells. As the combustion front advances, heat liberated by the front results in the vaporization of oil from a high temperat-ure zone preceding the front. Cracking of oil, Aformation of coke or heavy liquid hydrocarbons may also occur. Oil vapors formed by the process are carried forward with combustion products and are condensed in cooler portions ofthe reservoir. Heat transfer to cold oil in sections of a reservoir in front of and around the advancing high temperature zone leads to a reduction in viscosity of oil and facilitates its displacement from a reservoir. A mixture of oil and gases is withdrawn from a reservoir at the production well.

Although the present in-situ combustion methods of oil recovery are promising, there exist some disadvantages associated with them. One such disadvantage is that a combustion front and its associated high temperature region gravitates to the top of an oil-bearing formation as a result of gas-oil density differences. This leads to prematurel breakthrough of the combustion front into the production wells atan early stage in the process when only a small region of a reservoir has been burned through. After such breakthrough occurs, high temperature combustion products, oxygen and oil now into productionwells simultaneously, creating a danger of re in the production wells and also heat damage to associated well equipment. Severe corrosion may also occur of the equipment in the production wells.

Combustion breakthrough generally occurs at production wells near the top of a producing formation due to the gravity segregated induced overburning described above. Utilization in the combustion process of the injected oxygen thereafter depends primarily upon the diffusion of gaseous oxygen from the burned out area at the top of the formation down into the unburned zone. The distance through which the oxygen must diffuse increases as the thickness Iof the burned-out region increases and hence the portion of the injected oxygen utilized to support combustion generally decreases as an operation pro- I' eld experiments have shown that for a substantial part of the process, 50 percent or more of the injected oxygen is produced into producing wells at a time when oil recovery is still very low and incomplete. The economics of present in-situ combustion processes are so unfavorable because of this wasted air and attendant well maintenance problems associated with oxygen production that oil recovery operations must sometimes be discontinued at very low oil recoveries. The present invention is accordingly concerned with a method for increasing oxygen utilization in an oil reservoir so that more oil may be recovered economically. Reduced well-operating problems also result when less oxygen is produced.

As pointed out heretofore, the present invention is concerned with a two-phase operation wherein in an initial phase a bank of dense water is injected into a reservoir prior to starting a combustion phase. The water is preferably injected into the bottom of an oil-containing section through a well completed, for example, over only about 20-30 percent of the section-ie. the bottom 20-30 percent. The production wells are preferably and similarly completed such that the production of oil is from only the bottom of the section. The water channels along the bottom of the reservoir, thereby providing a reservoir path 0r zone of high mobility for the air and combustion products as compared to the portion or zone of the reservoir not contacted by the Water. By operating in this manner, overburning is prevented and the combustion zone advances down the center or near the bottom of the reservoir instead of the top.

Because the heat liberated by combustion is utilized more efficiently in heating a reservoir when burning is at the bottom or near the center rather than at the top (as occurs in the absence of the bank of heavy water), oil recovery in terms of the quantity of air injected is much more eicient and the total oil recovery is increased when a bank of dense water is used in accordance with this invention.

Further, by operating in the manner of the present invention, the time required to produce oil from a typical producing formation is also greatly reduced. This reduction results from the fact that the present invention deliberately exploits a marked difference between the upward and downward heat transfer rates directed from a combustion front within a formation. The difference appears to result primarily from the mechanism of vaporization and condensation of water and crude oil components. Lighter components will tend to rise in a formation while heavy oil in the upper part of the formation will tend to gravitate downward resulting in a mixture of the hot products with cooler ones. The result d then is that more oil is heated Jfaster and the oil production thereby greatly stimulated. In addition to exploitation time which is markedly decreased, there also results less time for heat losses to formations and strata which surround the oil-containing formation or stratum. With that approximately 1/2 pore volume ofdense water was injected prior to air injection and ignition.

The water was weighted Vby sodium bromide dissolved in the water such that the water density was .33 gram per cubic centi` meter greater than the oil in place.

The test core was unconsolidated sand packed in a 101/2 inch long, 31/2 inch inside diameter steel cylinder lying with its long axis horizontal. The permeability of the packed sand wasapproximately 220 darcies. The test core contained ll percent pore volume c'onnate water and 89 percent 420 centipose crude oil bottoms. Injection was into the bottom of the sand pack, and production was from the bottom of the sand pack.

The results of the tests and a better understanding of this invention are forthcoming by referring to the` accompanying figures that form a part of this application.

FIGURE 1 compares results obtained by a conven- Y tional in-situ oil recovery process with the results obtained by the present invention.

FIGURE 2 is a cross-sectional view through a portion of the earth showing diagrammatically the operation of a preferredembodiment of the invention.

Referring to FIGURE 1, the correlations or curves A and B indicate oil recovery as a percent of the amount of oil initially present in the test core versus cumulative gas-oil ratio (GOR) in thousands of standard cubic feet of produced gas per barrel of produced oil (m.s.c.f./bbl.). Curve A represents the oil recovered from the test core when using a conventional forward-drive type in-situ combustion oil recovery process.

Curve B indicates the oil recovered when using Vthe present invention. Recoveries are shown to be substantially higher and cumulative gas-oil `ratios much lower when a bank of dense water is used as required by the invention. When the test core was examined after each of the tests, a pronounced overburn (a burn only at the top of the core) was `found when the water was not used as compared to a center burn in the test in which the water bank was used.

In applying this technique to the field, scaled tests may be used as desired to determine both the density in-V crease of water and the amount of water required to achieve a center burn. However, for oil reservoirs where the preferred techniques of bottom injection and bottom production are used, only a very small density difference between the oil and water will be required as water will start and remain at the bottom of the formation.y

The quantity of water injected need not exceedgenerally about 0.5 pore volume of the portion of a reservoir under exploitation. It is desirable, however, that sufficient water be injected to cause water breakthrough into the production wells. With increasingly viscous oils, less and less water will be required; and in some reservoirs as little as v0.03 to 0.10 pore volume should suffice. i

Water of increased density may be obtained in a variety of ways, including the evaporation of sea water or brine in open pits. Saturated oil eld brine will be adequate for most reservoirs-wiz., in the case of a typical heavy crude oil in a reservoir at 140 F., it will provide d, a density differential about equal'to thatfof the example described earlier.

A number of water soluble salts may be used to increase the density of water; and sodium bromide and sodium chloride have already been named. Calcium chloride appears especially attractivefor the purpose. Increased density could also be achieved by adding a dense liquid miscibley withA water.Vv Ank example is ethylene bromohydrin which is dense and Soluble in.v water but not in petroleum.' f i Referring'to FIGURE 2, the legend 1 designates a subterranean oil-bearing formation interposed between relatively impervious strata 2 and 3. Strata 2` and 3 may be shales or other relatively impermeablematerials. VWells 7 and 8 extend down from the surface of the earth 4 and are completed in the lower portiony of formation 1. Casing strings 5 and 6 are cemented in wells 7 and 8, respectively. The annulus between each string of casing and its surrounding borehole is'sealed withce'ment or other sealing material. in a conventional manner. Each string of `casing is also perforated-note perforations 14 and IS-Whereby injected fluids may pass through the perforations between the wells and the oil-bearing formation. Control of the injection and production points for iluids into and out of the yformation 1 is thereby effected. Upon completionv of the wells 7 and S, water is pumped into the formation 1 and through well 7, preferably until breakthrough occurs into well 8. As noted i earlier, less than 0.5 pore volume of water need generally be pumped into any given formation. If the oil inV place has a density under reservoir conditions greater than the water, the density of the water should be increased until the reverse is true. This may be kdone by adding watersoluble salts orr equivalent materials to the water,

It will be recognized that water injection until breakthrough'need not always be practiced-eg. when one can be reasonably certain the Water has penetrated almost to the production well.V However, actual breakthrough is desirable, since it is a positive indication that a communicating zonefor example, zone9 in FIG. 2-has been established ybetween'the input and output wells.

Following breakthrough, iii-situ` combustion may Vbe started and practiced in a conventional manner. .For example, chemicaL-electrical `or other ignition techniques may be used. Also, forward or reverse combustion, or combinations thereof, may be employed. In any event an oxygen-containing gas such as air or substantially pure oxygen is injected through one or more input wells toward one or more output wells so as to propogate a combustion front from one set of the wells toward theV other set. Oil isrecovered from the output wells and removed to the surface in any suitable, conventional manner.

The surface equipment which would normally be employed in injecting or producing uids via wells penetratingv an oil reservoir is notfshown. The inclusion of such equipment is not considered essential vfor the purpose of describing this invention.

What is claimed is:

l. A method for the recovery of oil from an underground oil-bearing formation penetrated by an input well and a spaced output well which comprises: injecting water having a density greater than said oil through said input well into only a lower portion of said formation, continuing the injection of such Water `until waterk breakthrough Voccurs in said output well, thereafter initiating in-situ combustion in only said lower portionY of said formation about said input well and injecting an oxygencontaining gas through said input well into only'saidlower portion of said formation to support said combustion and to drive oil from said formation toward saidkoutput well.

2. A method for the recovery of oil from an underground oil-bearing yformation penetrated by an input well and ,a spaced output well which comprises: establishing communication between each said well and the lower portion of said formation, injecting water having a density greater than said oil into only said lower portion of said formation through said input well, continuing the injection of said water until the portion of the formation contacted by water extends a sutiicient distance between the wells to enable gas to iiow between said wells preferentially through said lower portion of said formation in comparison with the remainder of the formation, thereafter initiating an in-situ combustion within said lower portion of said formation in the vicinty of one of said wells, and injecting an oxygen-containing gas through said input well into only said lower portion of said formation to support said combustion and to drive oil from said formation into said output well.

3. A method of recovering oil from an underground oil-bearing formation penetrated Iby spaced input and output wells which comprises: injecting water having a density greater than the density of said oil into said lower portion only of said formation through an input well, the amount of such water so injected being less than 0.5 pore volume of the portion of the formation to be explotted between said input well and said output well but suicient to enable gas injected into said lower portion of said formation through said input well to travel through said lower portion of said formation in preference tothe remainder of said formation toward an output well, thereafter initiating insitu combustion within said lower portion only of said formation about said input well, and injecting an oxygen-containing gas through said input well into said lower portion only of said formation to support said combustion and to drive oil through said formation toward said output well.

4. A method as dened in claim 3 in which said lower portion of said formation comprises between 20 and 30 percent of said formation.

References Cited in the file of this patent UNITED STATES PATENTS 2,642,943 Smith June 23, 1953 2,780,449 Fisher et al Feb. 5, 1957 2,796,132 Bruce lune 18, 1957 2,874,777 Tadema Feb. 24, 1959 2,901,043 Campion et al Aug. 25, 1959 3,024,841 Willman Mar. 13, 1962 OTHER REFERENCES Morse, R. A.: Trends in Oil Recovery, Producers Monthly, pp. 18-21, February 1960. 

1. A METHOD FOR THE RECOVERY OF OIL FROM AN UNDERGROUND OIL-BEARING FORMATION PENETRATED BY AN INPUT WELL AND A SPACED OUTPUT WELL WHICH COMPRISES: INJECTING WATER HAVING A DENSITY GREATER THAN SAID OIL THROUGH SAID INPUT WELL INTO ONLY A LOWER PORTION OF SAID FORMATION. CONTINUING THE INJECTION OF SUCH WATER UNTIL WATER BREAKTHROUGH OCCURS IN SAID OUTPUT WELL, THEREAFTER INITIATING IN-SITU COMBUSTION IN ONLY SAID LOWER PORTION OF SAID FORMATION ABOUT SAID INPUT WELL AND INJECTING AN OXYGENCONTAINING GAS THROUGH SAID INPUT WELL INTO ONLY SAID LOWER PORTION OF SAID FORMATION TO SUPPORT SAID COMBUSTION AND TO DRIVE OIL FROM SAID FORMATION TOWARD SAID OUTPUT WELL. 